1. Field of the Invention
The present invention relates to a color image forming apparatus of an electrophotographic process such as a color printer or a color copying machine, and more particularly to a control of characteristics of a density or a gradation of an image in the apparatus.
2. Related Background Art
In recent years, demand has been growing for a higher quality image that is outputted from a color image forming apparatus employing an electrophotographic process, an ink jet process, or the like, such as a color printer or a color copying machine. In particular, a gradation of an image density and its stability greatly affect judgment as to whether a human considers an image to be good or bad.
However, if respective parts of the color image forming apparatus vary according to environmental variation or long-term use, the density of an obtained image varies as well. Especially the color image forming apparatus of an electrophotographic process may lose a color balance because the density of the obtained image varies due to even slight environmental variation, thereby making it necessary for the color image forming apparatus to include means for maintaining constant characteristics of a relationship between a density and a gradation all the time. Therefore, several kinds of exposure amounts according to an absolute humidity, process conditions including a developing bias, and gradation correcting means including a look-up table (LUT) are provided for toner of each color. Based on the absolute humidity measured by a hygrothermal sensor, the process conditions at that time and optimal values for gradation correction are selected.
Also, in order to obtain the constant characteristics of the relationship between a density and a gradation even upon variation of the respective parts of the color image forming apparatus, a density detecting toner patch is formed using the toner of each color on an intermediate transferring body, a drum, or the like. A density of the unfixed toner patch is then detected by an unfixed toner density detecting sensor (hereinafter, referred to as xe2x80x9cdensity sensorxe2x80x9d). Density control is performed by feeding back the detection results to the exposure amounts and the process conditions including a developing bias, thereby attaining a stable image.
The above-mentioned density control using the density sensor is performed by forming a patch on an intermediate transferring body, a drum, or the like for detecting the density, but not for controlling variation in the color balance of the image transferred onto and fixed to a transfer material. The color balance also varies according to transferring efficiency upon transferring of a toner image onto a transfer material, and according to heating and pressurizing upon fixing. The variation cannot be coped with by the density control using the density sensor.
It is therefore conceivable to provide a color image forming apparatus equipped with a sensor for detecting a patch color on a transfer material (hereinafter, referred to as xe2x80x9ccolor sensorxe2x80x9d). The color image forming apparatus is adapted to form on a transfer material a gray gradation patch using black (K) (hereinafter, referred to as xe2x80x9cgray gradation patchxe2x80x9d) and a gray gradation patch in process print using color mixture of cyan (C), magenta (M), and yellow (Y) (hereinafter, referred to as xe2x80x9cprocess gray gradation patchxe2x80x9d) and to compare colors of the two patches after the fixing, thereby enabling an output of such a CMY mixing proportion as to cause the process gray gradation patch to be of an achromatic color.
According to this color image forming apparatus, the detection results are fed back to exposure amounts and process conditions of an image forming part, a color matching table for converting an RGB signal from an image processing part into a color gamut of a color image forming apparatus, a color separation table for converting an RGB signal into a CMYK signal, a calibration table for correcting characteristics of a relationship between a density and a gradation, and the like. Accordingly, control of a density or chromaticity of an final output image formed on the transfer material can be performed.
The control can be performed similarly by detecting the output image from the color image forming apparatus using an external image reading apparatus, a chromascope, or a densitometer. However, the above-mentioned method using the color sensor is superior in that the control completes only in a printer. The color sensor includes, for example, a light emitting element that uses three or more light sources having different emission spectra such as red (R), green (G), and blue (B). Alternatively, the color sensor includes a light emitting element that uses a light source emitting a white color (W), and a light receiving element on which three or more kinds of filters having different spectral transmittances such as red (R), green (G), and blue (B). By the above arrangement, three or more different kinds of outputs including an RGB output can be obtained.
According to a conventional image forming apparatus, the above-mentioned control using the color sensor has been performed at a single conveying speed, that is, in a standard speed mode suitable for conveying a sheet of plain paper. However, in different speed modes such as a so-called half speed mode suitable for conveying a cardboard and a low speed mode suitable for effecting a high gloss, there occur differences in fixing property of toner (a gloss of a transfer material surface) and transferring efficiency (an amount of toner transferred onto a transfer material) between each mode, so that the color balance of the final output image may vary.
The present invention has been made in view of the above circumstances, and therefore has an object to provide a color image forming apparatus that uses a combination of a color sensor and a density sensor, in which a density is controlled at each conveying speed of a transfer material, thereby making it possible to attain a stable color balance at any conveying speed.
It is another object of the present invention to provide a color image forming apparatus, including:
an image processing part that is suitable for processing image data based on a calibration table;
a plurality of image forming parts that are suitable for forming an image using different colors from each other based on the image data outputted from the image processing part;
a conveying part that can convey at any one of a plurality of speeds a transfer material onto which the image formed by the plurality of image forming parts is transferred;
a fixing part that is suitable for subjecting the transfer material conveyed by the conveying part to fixing processing;
a first detecting part that is suitable for detecting a density of a patch formed by the plurality of image forming parts, the patch being unfixed;
a second detecting part that is suitable for detecting a chromaticity of the patch formed by the plurality of image forming parts, the patch being fixed; and
a setting part is suitable for revising the calibration table of the image processing part based on a detection result of the first detecting part and a detection result of the second detecting part, in which:
the image processing part stores the calibration table corresponding to each of the plurality of speeds; and
the setting part sets the calibration table corresponding to each of the plurality of speeds based on the detection result of the first detecting part and the detection result of the second detecting part.
It is further another object of the present invention to provide a control method for a color image forming apparatus that includes:
an image processing part that is suitable for processing image data based on a calibration table;
a plurality of image forming parts that are suitable for forming an image using different colors from each other based on the image data outputted from the image processing part;
a conveying part that can convey at any one of a plurality of speeds a transfer material onto which the image formed by the plurality of image forming parts is transferred; and
a fixing part that is suitable for subjecting the transfer material conveyed by the conveying part to fixing processing, the image processing part storing the calibration table corresponding to each of the plurality of speeds, the control method including:
a first detecting step of detecting by a first detecting part a density of a patch formed by the plurality of image forming parts, the patch being unfixed;
a second detecting step of detecting by a second detecting part a chromaticity of the patch formed by the plurality of image forming parts, the patch being fixed; and
a setting step of setting the calibration table of the image processing part based on a detection result of the first detecting part and a detection result of the second detecting part,
in which the setting step includes a step of setting the calibration table corresponding to each of the plurality of speeds based on the detection result of the first detecting part and the detection result of the second detecting part.
Other objects, structures, and effects of the present invention will become apparent upon reading and understanding the following detailed description with reference to the accompanying drawings.